Dental superstructure with curved channel and method of forming a curved channel therein

ABSTRACT

A dental superstructure and a method of forming a curved channel therein for a fixing element by using a computer-controlled device. A dental superstructure is fixed, preferably through an intermediate piece (interface), to a dental implant integrated in the jaw bone by means of a fixing element (typically a screw) through an outlet provided with a shoulder. To allow the insertion of the fixing element, a channel is formed in the superstructure between an inlet formed in the superstructure at its side facing the oral cavity and the shoulder of the outlet. The outlet having a diameter corresponding to a threaded part of the fixing element is usually formed in a separate working process.

This is a continuation-in-part of application Ser. No. 14/349,324 which is the national stage of international application no. PCT/HU2012/000026, filed Apr. 12, 2012, the entire disclosure of which is hereby incorporated by reference herein.

TECHNICAL FIELD

The invention relates to a dental superstructure and a method of forming a curved channel therein for a fixing element by using a computer-controlled device.

A dental superstructure is fixed, preferably through an intermediate piece (interface), to a dental implant integrated in the jaw bone by means of a fixing element (typically a screw) through an outlet provided with a shoulder. To allow the insertion of the fixing element, a channel is formed in the superstructure between an inlet formed in the superstructure at its side facing the oral cavity and the shoulder of the outlet. The outlet having a diameter corresponding to a threaded part of the fixing element is usually formed in a separate working process.

BACKGROUND ART

In dentistry, there is a need for replacing missing teeth by an attractive and aesthetic prosthesis. To satisfy this need, an artificial dental root (called implant) made of tissue-friendly material has been developed. The artificial dental root is implanted into the jaw bone in order to keep the artificial tooth or teeth firmly in position. In most cases, the material of the implant is pure unalloyed titanium. The artificial tooth or the set of teeth is screwed to the dental implant through a screw channel. In the prior art, the insertion direction of the screw and the screwing direction were the same, consequently the bore-hole passed through the outer, visible surface of the tooth. This made covering of the screw channel rather difficult. Therefore, a dental system is required in which the opening of the screw insertion channel is formed on a surface of the tooth that can not be seen from outside of the mouth.

Patent application EP2289461A1, titled “Dental System,” describes a dental superstructure in which a screw member is inserted into an aperture of a screw channel and a screw member seat with a second aperture for providing support to the head of the screw member during fixation of the dental superstructure to a spacer element or an implant through the second aperture. At least one part of a center line of the screw channel and a center line of the second aperture do not coincide.

In this “Dental System” solution, forming the channel is rather complicated. On the one hand, insertion of the screw is complicated because of the varying diameter of the channel, and on the other hand, the dental superstructure is unreasonably weakened.

U.S. Pat. No. 5,116,225 describes an angulated abutment system for affixing a dental prosthesis to an anchor implanted in the jaw bone. The dental prosthesis can be mounted axially offset from the axis of the implant. Two components of the abutment system allow the dental prosthesis to be adjusted in small angles of rotation.

This solution also takes measures to fix the superstructure from its side facing the oral cavity, but it uses a number of components, the production of which is complicated. Although the channel is formed to be straight, providing an opening from the direction of the dental implant is complicated because of the many intermediate component parts.

Patent application WO 2008/138852 describes an adapter for a dental implant with a conical connection recess in its upper part. The adapter comprises a threaded part for connection with the dental implant and a conical main body corresponding to a conical connection recess of the implant, a tool grip portion allowing for attachment of the adapter to the implant by using a tool, and a connection recess adapted for connection to a spacer element or a dental superstructure.

In the above solution, the spacer is provided with outer and inner threads and on the one hand, it is fixed in the implant, and on the other hand, the superstructure is fixed in it.

Typically, this type of spacer is not used any longer since in recent times superstructures have been screwed directly to the dental implants. For the sake of proper positioning of the superstructure, a suitable interface may be installed between the dental implant and the superstructure.

SUMMARY OF INVENTION

The aim of the present invention is to provide a dental superstructure having a simpler channel formed therein as compared to the prior art solutions, which can be implemented easily with today's technical background, which is efficient and the preparation of which can be automatized. It helps dentists in working faster thereby causing less inconvenience to patients.

It has been realized that by forming a screw channel extending along a circular center line in one process, the screw used for fixing the superstructure can be inserted easily into the channel formed in the superstructure. Installation may be made easier by forming a channel with a slightly spiral center line along an imaginary spherical surface. This can be done in one working process by using a milling machine controlled by a computer. If a spherical cutter is used, the shoulder surface of the shouldered outlet facing the curved channel can be formed as a regular segment of a sphere into which the head of the fixing element can be fitted perfectly.

The bore-hole of the outlet makes it possible for the fixing element, with its lower end engaging with the shoulder being also shaped as a segment of a sphere, to take the proper position when the superstructure is fixed to a dental implant. Additionally, if the shank of the fixing screw is made thinner above the threaded part and the interface is provided with a threaded bore-hole corresponding to the threads of the screw, then before the superstructure is placed in it final position, after installation of the screw, the interface can be driven through the threaded part of the screw. In this manner the fixing element cannot fall out. In known solutions, these interfaces have been only placed on the fixing element entailing the possible loss of the individual components. In case of a lower denture, keeping the interface in its proper place was problematic while in case of an upper denture, temporary fixing of the screws was problematic.

According to a first aspect of the present invention, a dental superstructure with a curved channel for receiving a fixing element is provided. The superstructure can be fixed to a dental implant integrated in the jaw bone by means of a fixing element through a shouldered outlet formed in the superstructure. To allow the insertion of the fixing element, a channel is formed in the dental superstructure between an inlet formed in the superstructure at its side facing the oral cavity and the shoulder of the outlet. Advantageously, the outlet is bored into the superstructure from the side of the dental implant in a separate working process, before or after forming the channel, in such a manner that it has a diameter corresponding to the threaded part of the fixing element. Alternatively, the shoulder may be formed from the inside of the channel. The center line of the channel is shaped to at least partly follow a circular arc along an imaginary spherical surface. In one embodiment the circular center line transversally diverges from a central plane of the circular arc by at most ±5%, thus the center line of the respective curved channel section may represent a three-dimensional spiral, i.e. a right-handed or left-handed helical line instead of a two-dimensional arc. Furthermore, the curved channel is shaped in such a manner that its cross-sections in the planes perpendicular to its center line are circles of the same diameter with their centers residing on the center line.

According to a second aspect of the present invention, a dental superstructure with a channel extending through the superstructure between a first opening and a second opening of the superstructure is provided, wherein the channel has a continuous center line, and wherein the channel is formed of a single arcuate channel extending between said first and second openings, wherein the center line of the arcuate channel runs on an imaginary spherical surface. Between the first opening and a location adjacent to the second opening, the channel has a circular cross-section with a first cross-sectional area in any plane perpendicular to its center line, and a shoulder is formed at the second opening of the superstructure so that the second opening has a second cross-sectional area that is smaller than said first cross-sectional area.

Advantageously, the fixing element is a screw having a shank portion between the screw head and the threaded part, the diameter of which shank is smaller than the diameter of the threaded part.

Preferably, the dental superstructure is fixed to a dental implant using an interface by means of a screw wherein threads corresponding to the threaded part of the screw are formed in the interface.

Advantageously, the channel is formed by means of a spherical cutter, the shank diameter of which is at most 60% of the cutter head diameter.

Also, according to the present invention, a method of forming a channel for a fixing element in a dental superstructure by using a computer-controlled device is provided. By means of a fixing element introduced through an inlet hole of the channel, the superstructure can be fixed, via a shouldered outlet formed in the superstructure, to the dental implant integrated into the jaw bone. Advantageously, the outlet is bored into the superstructure from the side of the implant in a separate working process, before or after forming the channel, in such a manner that it has a diameter corresponding to the diameter of the shank of the fixing element. Between the side of the superstructure facing the oral cavity and the shoulder of the outlet, a channel having a circular cross-section is formed by means of a spherical cutter. In this process, a bore-hole having a cross-section corresponding to the cross-section of the channel is formed in the superstructure, wherein the center line of the channel at least partly forms a circular arc along an imaginary spherical surface. In one embodiment the circular center line transversally diverges from a central plane of the curved channel by at most ±5%, thus the center line of the respective curved channel section diverges from the central plane of the circular arc by at most ±5%. It means that in a certain case, the center line of the channel is formed to be helical.

Further, the channel may be shaped in such a manner that its cross-sections in the planes perpendicular to its center line are circles of the same diameter with their centers residing on the center line.

Advantageously, the channel is formed by means of a spherical cutter, the shank diameter of which is at most 60% of the cutter head diameter.

Preferably, the dental superstructure is fixed to a dental implant by means of a fixing element, which has a shank between its head and its threaded part, said shank portion having a smaller diameter than the diameter of the threaded part.

Advantageously, the superstructure is fixed to a dental implant through an interface, in which threads are formed corresponding to the threaded part of the screw.

BRIEF DESCRIPTION OF DRAWINGS

A detailed description of the invention will be given with reference to the accompanying drawings in which:

FIG. 1 is a side view showing the cross-section of a blank dental superstructure;

FIG. 2 is a cross-sectional side view showing the a metal-sintered dental superstructure, into which a spatial mesh framework of the channel and the outlet is inserted during the method of forming a channel is shown;

FIG. 3 is a cross-sectional side view of the superstructure showing a number of positions of the spherical cutter during process of forming a channel;

FIG. 4 is a cross-sectional side view of the superstructure showing the process of inserting the fixing element into the channel;

FIG. 5 is a side view of a possible embodiment of a fixing element;

FIG. 6 shows the cross-section of a possible embodiment of an interface;

FIG. 7 is a cross-sectional side view of a superstructure and an interface showing the assembled superstructure before it is fixed to a dental implant;

FIG. 8 is a rear view of the cross-section of an element of the superstructure in case of a helically formed channel;

FIG. 9 is a partial cross-sectional side view of the superstructure mounted on a dental implant with an inserted interface, wherein the fixing element is depicted in side view;

FIG. 10 shows the side view of a possible embodiment of a spherical cutter.

FIG. 11 is a cross-sectional side view showing a superstructure with a single curved channel and a first type of shoulder according to the second aspect of the present invention.

FIG. 12 is a cross-sectional side view showing a superstructure with a single curved channel and a second type of shoulder according to the second aspect of the present invention;

FIG. 13 is a cross-sectional side view showing a superstructure with a single curved channel and a third type of shoulder according to the second aspect of the present invention;

FIG. 14 is a cross-sectional side view of the superstructure showing a number of positions of a spherical cutter during the process of forming the channel, according to the second aspect of the present invention.

FIG. 15 is a cross-sectional side view of the superstructure showing the process of inserting a fixing element into the channel, according to the second aspect of the present invention.

FIG. 16 is a cross-sectional view of the superstructure mounted on a dental implant with an inserted interface, wherein the fixing screw is shown in side view, according to the second aspect of the present invention.

FIG. 17A schematically illustrates the geometry of various arcuate center lines according to the first and second aspect of the present invention.

FIG. 17B illustrates a perspective view of a dental superstructure with a helical channel according to the second aspect of the present invention.

DESCRIPTION OF EMBODIMENTS

With the present invention, a channel 7 is formed in a dental superstructure 1 for an aesthetical attachment of the dental superstructure shown in FIG. 1 to a dental implant. An inlet 6 is formed on a side of the superstructure 1 facing the oral cavity. (In case of a superstructure 1 comprising a number of false teeth it is not necessary to form a channel 7 in each of the false teeth.) In this manner, when the superstructure 1 and the dental implant 3 are fixed together by means of a fixing element 2, the filling material used for covering the inlet 6 of the channel 7 will not be seen after the superstructure 1 is mounted on. In addition, the filling material will not appear on the occlusal surface of a false tooth. Typically, forming this type of channel 7 can be important mainly in the case of front teeth (FIG. 9).

Advantageously, in the simplest and most efficient way, the channel 7 is formed in the superstructure 1 by a computer-controlled device. The fixing element 2 may be inserted into the channel 7 formed in superstructure 1. The channel 7 is provided with an inlet 6 facing the oral cavity. It has a diameter large enough to receive a fixing element 2, which, in this example, is an Allen screw (FIG. 5). The fixing element 2 comprises a head 10, a threaded part 11 and a shank portion 12. In the superstructure 1, the channel 7 extends to the shoulder 4 of outlet 5. The shoulder 4 provides a seat for the head 10 of the fixing element 2. The threaded part 11 and a major part of the shank portion 12 of the fixing element 2 are driven out through the outlet 5 of the channel 7 before the superstructure 1 is mounted on. The head 10 of the fixing element 2 is seated on the inner side of the shoulder 4, which may be shaped as a segment of a sphere. The surface of the head 10 of the fixing element 2 facing the shoulder 4 may also be shaped as a segment of a sphere. The outlet 5 is bored into the superstructure 1 from the side of the dental implant 3 in another working process, before or after forming the channel 7, in such a manner that it has a diameter corresponding to the diameter of the threaded part 11 of the fixing element 2.

According to the present invention, the center line 8 of the channel 7 forms a part of a circular arc 9. The circular arc 9, defining the center line 8 of the channel 7, may also be formed in such a manner that it diverges from its own plane by a few degrees, at most by ±5 degrees. In this manner, a slightly diverging spiral path is produced along the periphery of the circular arc 9 (FIG. 8). That is, advantageously, the center line 8 represents a three-dimensional spiral line, a right-handed or left-handed helical line instead of a two-dimensional arc. The cross-section of the channel 7 is essentially constant. It narrows only at the outlet 5 to have a diameter corresponding to the diameter of the outlet 5 (FIG. 7).

The fixing element 2 is a screw having a shank portion 12 between its head 10 and its threaded part 11, wherein the diameter of said shank portion 12 is smaller than the diameter of the threaded part 11.

In many cases, an interface 13 (FIG. 6) is located between the superstructure 1 and the dental implant 3, which are fixed together by means of a fixing element 2. In order to make mounting of the superstructure 1 onto the dental implant 3 easier, threads corresponding to the threaded part 11 of the fixing element 2 are formed in the interface 13. Thus, after insertion of the fixing element 2 into the superstructure 1, the interface 13 can be driven through the threaded part 11 of the fixing element 2 onto the shank portion 12. In this manner both the interface 13 and the fixing element 2 are prevented from falling out when the superstructure 1 is mounted on a dental implant 3. At the same time, insertion of the fixing element 2 into the dental implant 3 is not inhibited by the interface 13.

According to a second aspect of the present invention, a dental superstructure 101 is provided with a single arcuate channel 107 extending between a first opening 106 (corresponding to the inlet 6 of the superstructure 1 of the first aspect) and a second opening 105 (corresponding to the outlet 5 of the superstructure 1 of the first aspect) as shown in FIG. 11. In this type of superstructure 101 the arcuate channel 107 extends up to the second opening 105 and therefore there is no straight channel section within the superstructure 101 like in the above described superstructure 1 of the first aspect. As shown in FIG. 11 the arcuate channel 107 terminates at a shoulder 104 which also defines the second opening 105 at its outer side.

Between the first opening 106 and a location ‘A’ adjacent to the second opening 105, the arcuate channel 107 has a circular cross-section with a first cross-sectional area in any plane perpendicular to its center line 108. Between the location ‘A’ and the second opening 105, the cross-section of the arcuate channel 107 is narrowing towards the second opening 105. The second opening 105 has therefore a smaller cross-sectional area than that of the other part of the channel 107. The second opening 105 has a cross-sectional area that fits to the diameter of the threaded shank of a fixing screw used for fixing the dental superstructure 101 to an implant.

The tangent line t of the channel 107 touching the circular center line 108 at the location A may define a wide range of angles with respect to the normal n of the plane of the second opening 105 (i.e. a line perpendicular to the plane of the second opening). As shown in FIG. 11, the tangent t defines an acute angle with the normal n. In another example embodiment shown in FIG. 12, the tangent t′ is parallel to the normal n′ of the plane of the second opening 105, in particular it coincides with a center line of the second opening 105.

Although it is particularly preferred that the arcuate channel is formed by means of a sperical cutter head resulting in the shoulder 104 having an inner side surface shaped corresponding to a segment of a sphere, as shown in FIGS. 11 and 12, the arcuate channel may also be formed using a cutter with a head shaped differently, for example by means of a tapered cutter head. In an alternative embodiment of the superstructure 101, a shoulder 104′ is formed with a tapered inner side surface as shown in FIG. 13.

Generally, the center line 108 of the arcuate channel 107 runs on an imaginary spherical surface. In one embodiment of the superstructure 101, the center line 108 of the channel 107 is circular and running within a plane. In another embodiment, the center line 107 of the arcuate channel 107 may run on an imaginary spherical surface so that along its path the center line 107 has a transversal deflection of at most ±10° with respect to a circular arc defined between the center of the first opening 106 and the center of the second opening 105 of the superstructure 107.

In FIG. 15 is a cross-sectional side view of the superstructure 101 showing the process of inserting a fixing element 102, typically a screw, into the arcuate channel 107, according to the second aspect of the present invention. When the fixing element 102 is a screw, the fixing element 102 comprises a head 110, a threaded part 111 and a shank portion 112 between its head 110 and its threaded portion 111. The head 110 of the fixing element 102 may be shaped as a segment of a sphere and the inner side of the shoulder 104 may also be shaped as a segment of a sphere as shown in FIGS. 11 and 12. Due to the matching shapes of the screw head 110 and the inner side of the shoulder 104, the head 110 of the fixing element 102 can be seated onto the shoulder 104 in a form-fitting manner. When the inner side of the shoulder 104′ is shaped to form a tapered surface as shown in FIG. 13, the screw head of the fixing element should also be shaped to have a tapered lower portion to firmly fit to the inner side surface of the shoulder 104′.

FIG. 16A shows the superstructure 101 when directly attached to a dental implant 103. In this case, the bottom of the superstructure 101 seats on the upper surface of the implant 103 and the head 110 of the threaded fixing screw 103 firmly secures the superstructure 101 to the implant 103.

Alternatively, as shown in FIG. 16B, an additional interface 113 may also be interposed between the superstructure 101 and the dental implant 103 to prevent the fixing screw 102 from falling out from the superstructure 101 during its installation process or to allow a better positioning of the superstructure 101 with respect to the dental implant 103.

FIG. 17A schematically illustrates the geometry of various arcuate center lines 108 and 108′. The center line 108 extends along a circular path, whereas the center line 108′ follows a helical path along the surface of an imaginary sphere S. The center lines 108, 108′ have a starting point corresponding to the inlet of the superstructure, and an end point A adjacent to the inlet of the superstructure. The greatest transversal deflection of the helical center line 108′ with respect to the circular center line 108 can be measured along the latitude L of the sphere S, wherein the radii r1 and r2 define an angle α of at most 10°. Since the deflection of the helical line 108′ may be formed also in the opposite direction along the latitude L, the transversal deflection may be formed within a range of ±10° with respect to the circular center line 108. It is noted that the same geometry may be applied also to the curved channel section of the channel 7 according to the first aspect of the present invention.

FIG. 17B illustrates a perspective view of a dental superstructure according to the second aspect of the present invention, wherein the arcuate channel 107 is shaped to follow a helical center line.

Now the milling process of forming the screw channel in the superstructure will be introduced in more detail with regard to the channel according to the first aspect of the present invention. As shown in FIG. 3, to form the channel 7, a spherical cutter 14 is used. The cutter head is not necessarily ball-shaped, it can be dome-shaped, or it may have the shape of a segment of a sphere. The cutter head 17 of the spherical cutter 14, used in the present invention, is preferably ball-shaped. In the present example, the diameter 16 of the shank 15 of the spherical cutter 14 is 30% of the diameter of the cutter head 18 (FIG. 3). The end 20 of the shank 15 can be fixed in the computer-controlled device (FIGS. 3 and 10). The path, along which the cutter head 17 advances, is also shown in FIG. 3, while FIG. 4 shows the travel of the fixing element 2 in the channel 7 during its insertion.

Dental implants can be produced with great accuracy in a highly productive metal sintering method. Much material and work can be saved during the manufacture if a spatial mesh framework 19 is formed in the space of the channel 7. The spatial mesh framework 19 ensures that a satisfactorily dense material can be built on the other parts of the implant 3. Considering that the material used for making the implant is very expensive, this solution may result in significant material savings. Between the frame elements of the spatial mesh framework 19, the space angle is 30 degrees. This framework ensures the required static structure and accuracy. Removal of these supporting elements can be done on completion of the metal sintering method as described previously.

According to the invention, the method of forming a channel 7 for receiving a fixing element 2 in dental superstructure 1 is accomplished by means of a computer-controlled device. During the method, a channel 7 with a circular cross-section is formed from the side of the superstructure 1 facing the oral cavity to the shoulder 4 of outlet 5 by means of a spherical cutter 14 (FIG. 3). To this end, a bore-hole having a cross-section corresponding to the cross-section of the channel 7 is formed in the superstructure 1, wherein the center line 8 of the bore-hole defined by a circular arc 9 in such a manner that radii running from the center of the arc 9 and pointing to given points of the center line 8 of the bore-hole diverge from the plane of the arc 9 by at most ±5 percent. It means that, in a certain case, the center line 8, as well as the channel 7 is formed to be helical (FIG. 8). The axis of the threaded bore-hole formed in the dental implantat 3 is tangent to the center line 8. Furthermore, the channel 7 is shaped in such a manner that the cross-sections in the planes perpendicular to its center line 8 are circles of the same diameter with their centers residing on the center line 8.

To form the channel 7, a spherical cutter 14 may be used. The diameter of the shank 15 of the spherical cutter 14 is at most 60%, preferably only 30% of the diameter of the cutter head 18.

The dental implant 3 can be made of any biologically compatible metal or ceramic material. Usually, the material of the superstructure 1 is zirconium (Zr), cobalt-chrome alloy, titanium, etc. Of course, any kind of material used in dental technology may be suitable.

The outlet 5 can be formed on the surface of the superstructure 1 facing the dental implant 3 by a simple boring process. To cut the channel 7, a high precision five-axis milling machine may be used with a spherical cutter 14, which is driven along a two- or three-dimensional (spiral) path. Shaping is carried out by means of a cutter-head 17 having a shank 15 with a diameter 16 smaller than the diameter of the cutter head 18. This makes it possible that concave, so-called undercut surfaces, can be shaped from the direction of work.

Then the fixing element 2 can be inserted easily into the channel 7.

FIG. 14 schematically illustrates the process of forming the arcuate channel 107 when a cutter 114 having a sperical cutter head 117 is used. The cutter head 117 is advanced within the body of the superstructure 101 along a circular path (or generally, along a path on an imaginary spherical surface) until its center reaches the location A of the circular center line 108. At this position of the cutter head 117, a portion of the cutter head 117 has overrun to outside the superstructure 101 thereby forming the shoulder 104 and the second opening 105.

It is noted that instead of a milling process, the curved channel may be formed in the superstructure through three-dimensional (3D) printing of the superstructure from an appropriate material. In this case the material of the superstructure is to be selected so that it is adapted for 3D printing, on the one hand, and it is certified for medical use, on the other hand. Such materials may include cobalt, chromium, titanium, zirconia specific and specific plastic materials, like polyetheretherketone (PEEK). Alternatively, a three-dimensional model of the superstructure with the curved channel may be printed using a non-medical material, and subsequently a mould may be formed around the model. The mould may then be used for casting a final superstructure of a material allowed for dental use.

According to an embodiment of the present invention, the screwing direction and the screw insertion direction are on the same arc. The two- or three-dimensional (spiral) path tangent to the central axis of the dental implant at the same time corresponds to the screw insertion direction. Consequently, any unnecessary procedure, which would weaken the dental implant, can be avoided and a statically stronger superstructure can be obtained as compared to the known superstructures, in which some portions of the channel were significantly widened. The embodiments according to the present invention provide a high-level solution both in terms of aesthetics and assembly technique. It is aesthetic since the inlet of the channel cannot be seen when the person wearing superstructure is talking. In respect of the assembly technique, the solution according to the present invention makes it possible for a dentist to screw a dental implant to its place by means of a device tilted towards the oral cavity. In this case, the opposing row of teeth does not interfere with the screwing operation. 

1. A dental superstructure comprising a channel extending through the superstructure between a first opening and a second opening of the superstructure, said channel having a continuous center line, wherein said channel is formed of a single arcuate channel extending between said first and second openings, wherein the center line of the arcuate channel runs on an imaginary spherical surface, wherein between the first opening and a location adjacent to the second opening, said channel has a circular cross-section with a first cross-sectional area in any plane perpendicular to its center line, and wherein a shoulder is formed at the second opening of the superstructure so that the second opening has a second cross-sectional area that is smaller than said first cross-sectional area.
 2. The dental superstructure of claim 1, wherein the center line of the channel is circular and running within a plane.
 3. The dental superstructure of claim 1, wherein the center line of said arcuate channel runs on said imaginary spherical surface so that along its path the center line has a transversal deflection of at most ±10° with respect to a circular arc defined between the first and second openings of the superstructure.
 4. The dental superstructure of claim 1 having only a single said channel.
 5. The dental superstructure of claim 1 wherein an inner surface of the shoulder is shaped as a segment of a sphere.
 6. The dental superstructure of claim 1 wherein an inner surface of the shoulder is shaped as a segment of a cone.
 7. A method of forming a channel in a solid dental superstructure, said method comprising the steps of: forming an arcuate channel in a solid superstructure using a milling tool with starting from a first opening and ending at a second opening of the superstructure, said arcuate channel having a center line running on an imaginary spherical surface, wherein between said first opening and a location adjacent to said second opening, the channel is formed with a circular cross-section having a first cross-sectional area in any plane perpendicular to its center line, and wherein a shoulder is formed at the second opening of the superstructure so that the second opening has a second cross-sectional area that is smaller than said first cross-sectional area.
 8. The method of claim 7, wherein the arcuate channel is formed to have a circular center line running within a plane.
 9. The method of claim 7, wherein the arcuate channel is formed with its center line running on said imaginary spherical surface so that along its path the center line has a transversal deflection of at most ±10° with respect to a circular arc defined between the first and second openings of the superstructure.
 10. A method of producing a dental superstructure having an arcuate channel therein, said method comprising forming a dental superstructure having an arcuate channel therein by means of three-dimensional printing, said arcuate channel having a center line running on an imaginary spherical surface, wherein between said first opening and a location adjacent to said second opening, the channel is formed with a circular cross-section having a first cross-sectional area in any plane perpendicular to its center line, and wherein a shoulder is formed at the second opening of the superstructure so that the second opening has a second cross-sectional area that is smaller than said first cross-sectional area. 